The invention relates to a vibration absorber comprising an absorber mass connected with a vehicle body by several arrangements that each comprise an absorber spring and an absorber damper.
In the case of vehicles without a stable roof bond (particularly convertibles and trucks) elastic torsional natural vibrations occur in the low frequency range. The reason for this is that the torsional rigidity of these vehicles is clearly reduced. The reduced torsional rigidity becomes noticeable by the occurrence of "trembling vibrations" which occur when driving over a rough road.
Vibration absorbers are known for avoiding the above-mentioned trembling vibrations in convertibles. These known vibration absorbers consist of a relatively heavy absorber mass which is connected with the vehicle body by means of four rubber bearing elements with hydraulic damping. However, this vibration absorber has the disadvantage that the natural frequency of the vibration absorber must be precisely adapted to the natural frequency of the torsional vibration of the vehicle. This adaptation requires that values for the rigidities of the absorber bearing be precisely obtained. It is also a disadvantage that a relatively heavy vibration absorber must be installed. Here, the frequency range covered by the vibration absorber is sufficiently large in order to accommodate the manufacturing tolerances of the vibration absorber as well as that of the vehicle.
It is an object of the invention to provide a vibration absorber which avoids the disadvantages occurring in this known state of the art and permits an effective damping of the torsional natural vibrations of the vehicle within a larger frequency range.
This object is achieved by having the absorber mass have one end arranged to be pivotable around a pivot with an opposite end supported on the vehicle body by way of at least one bearing arrangement and wherein either the absorber mass and/or the bearing arrangement are arranged to be adjustable with respect to the pivot.
For determining the natural frequency of the vibration absorber according to the invention, the vibration absorber is considered to be a pendulum. In the following equations,
c=the spring rigidity of the bearing arrangement (N/mm) PA0 m=the effective absorber mass (kg) PA0 .theta.=the moment of inertia of the absorber mass around its center of gravity (kg.times.mm.sup.2) PA0 a=distance between the joint and the bearing arrangement (mm) PA0 b=distance between the joint and the center of gravity (mm) PA0 f=natural frequency (l/s)
These quantities are related by the equation: ##EQU1## which can be rewritten as: ##EQU2##
The natural frequency of the vibration absorber can therefore be adjusted by a change of the effective torsional rigidity "c.times.a.sup.2 ", when the distance "a" is changed. In addition, the natural frequency of the vibration absorber can be adjusted by a change of the effective moment of inertia ".theta.+m.times.b.sup.2 ", when the distance "b" is varied.
The above-mentioned information shows that the adjustment of the natural frequency of the vibration absorber is possible at a constant torsional rigidity "c" and a constant absorber mass "m". The adjustment of the natural frequency of the vibration absorber takes place in a simple manner in that the absorber mass and/or the bearing arrangement is shifted with respect to the pivot. It is thus possible to optimally coordinate the vibration behavior of any vehicle. The frequency range which can be adjusted by the vibration absorber is at least as large as the range of dispersion of the torsional natural frequencies of the vehicles which occurs during series production. It is also advantageous in that the weight of the vibration absorber can be reduced to a minimum. The construction of the vibration absorber according to the invention, provides for sufficient stability so that no tilting of the vibration absorber is possible. In addition, it is possible to use a uniform vibration absorber of the same construction in the case of different vehicles.
The weight of the vibration can be further reduced by using a vehicle component as the absorber mass, instead of using a dead mass. The vehicle battery is particularly suitable for this purpose.
In addition to the above-mentioned direct shifting of the absorber mass for changing the distance "b", an additional mass can be shifted with respect to the pivot. It is also advantageous if the adjustment of the natural frequency of the vibration absorber can be adapted continuously to the respective torsional natural frequency of the vehicle. This is significant, because the torsional natural frequency of a vehicle can shift noticeably. Different torsional natural frequencies occur when the vehicle is driven either with an open top or with a closed top or with a hard top. In addition, the loading condition of the vehicle may also have an effect on the torsional natural frequency. The automatic adjustment of the natural frequency of the vibration absorber to the respective torsional natural frequency of the vehicle is achieved by installing the vibration absorber into a control circuit. A variable method of operation of the control circuit, as either a slow or a fast adaptation of the natural frequency of the vibration absorber to the respective torsional natural frequency of the vehicle can take place.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.